The present invention relates to a non-formaldehyde self-crosslinking latex and more particularly to a latex containing a starch-polymer graft modified by reacting with a glyoxal compound and having coatable viscosity, good storage stability and which provides a film having good tensile strength and elongation properties and which is suitable for use as a coating or a binder for non-woven fibers. This latex is water dispersible and stable in liquid form without the need of surfactants and upon drying and curing the starch is insolubilized and crosslinks providing good water resistance and wicking resistance.
Industrial starch may be utilized in a wide variety of applications including as coatings for paper or paper board. It desirably has good tensile and elongation properties, both under wet and dry conditions. Starch compositions may desirably be prepared in the form of coatable aqueous dispersions capable of being drawn down and cured on suitable substrates. The end user of these compositions may utilize them in a wide variety of applications by simply removing solvent to produce strong water resistant films.
A pre-gelatinized starch in liquid form would be desirable for certain end users as it would eliminate the need to handle dry starch and eliminate a cooking and dispersing step in the process. A problem with such pre-gelatinized starch is its stability over time. Because the product may spend many days in distribution and transportation, and because it sometimes is stored for a time prior to use at its ultimate destination, it is important that the product have a somewhat extended storage life wherein it maintains its desirable qualities and characteristics, particularly coatable viscosity. Prior art gelatinized starch products containing amylose tend to undergo a process known as retrogradation which results in the formation of a gel or precipitate. The viscosity also tends to thicken over time to a point which renders the products unusable for many commercial purposes. The problem is that by the time the pre-gelatinized starch product has been transported to its end user and prepared for use, it has often thickened to a point where it is no longer of a coatable viscosity. A preferred viscosity is about 350 centipoise or less. Viscosities above about 1000 centipoise are not readily flowable and are generally not usable in products such as coatings.
Non-woven fabrics cover a wide array of products. One of the non-woven bonding methods is to treat a staple or filament fiber sheet with an emulsion polymer containing a formaldehyde based crosslinking resin in order to impart water and solvent resistance. When the emulsion is dried and cured the polymer forms intimate bonding of the fibers. This polymer deposition modifies the strength, stiffness, environmental resistance, elongation and many other characteristics of the fiber fabric sheet. The fiber can be from a great variety of compositions, e.g. rayon, wood pulp (cellulose), cotton, nylon, polyester, glass and graphite. In the case of carded staple fiber the polymer usually contributes most of the strength and toughness character in the resulting non-woven. In wet-laid wood pulp fiber products the polymer is able to provide the non-woven strength and resistance to aqueous and solvent environments which the untreated non-woven would not have. In glass mat non-wovens traditionally bonded with a urea-formaldehyde resin, addition of emulsion polymers alters the toughness of the resulting non-woven. Even in filament or staple fiber non-wovens which are bonded by mechanical (i.e. needle punching) or thermal (e.g. spun bonded) techniques and are formed into useful non-woven fabrics without a chemical treatment, an additional application of an emulsion polymer can enhance or produce other valuable characteristics such as stretch resistance or non-slip character.
Melamine-formaldehyde resins do not build viscosity in the binder until cured, but they have the disadvantage of having an unpleasant odor and of releasing free formaldehyde. Curing with such resins involves the crosslinking of binder molecule with the methylol or methylated methylol group of the melamine resin. Free formaldehyde can be released either directly from the binder or mixture or when the coating is cured on the drying machine. The presence of even less than one percent of free formaldehyde, based on the total weight of the product, is undesirable, not only because of its objectionable odor, but because it is an allergen and an irritant, causing severe reactions in the operators who manufacture the binders and who treat and handle the non-woven fibers or fabrics.
The crosslinking of starches with multifunctional reagents which are reactive with starch hydroxyl groups is well known. Glyoxals and polyaldehyde compounds and resins have been previously utilized as crosslinkers. The simple mixing of glyoxal with a starch dispersion will provide a gel. U.S. Pat. No. 4,455,416 describes a paper coating containing starch binder and a cyclic urea/glyoxal/polyol condensate as an insolubilizer for the binder. As an insolubilizer the glyoxal condensate is inactive until the coating is applied and cured upon drying whereupon the glyoxal crosslinks the starch to impart water resistance. U.S. Pat. No. 3,740,391 describes the production of aqueous dispersions of a starch first reacted with an acrylamide and, subsequently, with glyoxal. U.S. Pat. No. 4,013,629 discloses a glyoxal binder system. U.S. Pat. No. 4,076,663 discloses a water absorbent resin prepared by polymerizing a polysaccharide with a water soluble monomer to which a crosslinking agent can be added during polymerization or to the reaction product. British Patent No. 2,017,124 discloses polysaccharides crosslinked with glyoxal. EPA 0223145 discloses emulsion polymerization of vinyl ester monomers in the presence of 1-10% of a hydrophobically-modified starch, wherein the emulsion may be crosslinked.